For example, lithium ion secondary batteries are well known as nonaqueous electrolyte secondary batteries. Lithium ion secondary batteries are excellent in energy density, output density, charge/discharge cycle characteristics, and the like as compared to other secondary batteries such as lead storage batteries. Thus, lithium ion secondary batteries are employed in mobile terminals such as smart phones, tablet terminals, and notebook PCs, and contribute to downsizing, weight reduction, and high performance of such terminals. On the other hand, as secondary batteries for electric automobiles and hybrid automobiles (secondary batteries for use in automobiles), lithium ion secondary batteries are not yet developed enough to deliver high performance in terms of output, time necessary for charging, and the like. Accordingly, aiming at high output and charging time reduction in nonaqueous electrolyte secondary batteries, studies for improving charge and discharge characteristics at high current density are underway.
A nonaqueous electrolyte secondary battery includes a pair of electrodes arranged via a separator and a nonaqueous electrolytic solution. The electrodes include a current collector and a mixture layer formed on the surface of the current collector. The mixture layer is formed by coating a slurry including an active material, a binder, and the like on the current collector, drying the slurry, and the like. In general, the obtained mixture layer is subjected to a treatment, such as compression, so that electrode density is increased, whereby electric capacitance of the battery is increased for use.
In the case of a lithium ion secondary battery as a nonaqueous electrolyte secondary battery, lithium ions move from the positive electrode to the negative electrode during charging, and move from the negative electrode to the positive electrode during discharging. When current density during charging and discharging increases, the amount of the lithium ions that move per unit time between the positive and negative electrodes increases. Lithium ions usually move in an electrolyte present in gaps between densely-filled active material particles, and therefore migrate and diffuse through extremely narrow and winding paths. Due to this, the increased current density during charging and discharging causes a phenomenon in which migration and diffusion of the lithium ions are delayed with respect to the amount of current and the ions are not supplied to the entire active material. The phenomenon particularly tends to occur on a current collector side. In addition, similarly in a nonaqueous solvent included in the electrolyte, the migration and diffusion of the lithium ions also become insufficient with respect to the amount of current, thereby causing a phenomenon (electrolytic solution depletion) in which a shortage of nonaqueous solvent molecules that are to be coordinated to the lithium ions on an active material surface occurs. As a result of these phenomena, a problem arises in that effective capacity becomes small when the current density increases.
Furthermore, in the negative electrode, when charge current density increases, some of the lithium ions cannot react with the active material and are deposited as metal lithium on the surface of the negative electrode, whereby negative electrode characteristics are deteriorated, causing a problem with reduced battery life.
In order to prevent the problems and enable charging and discharging at high current density, i.e., increase output density, it is effective to increase the speed and amount of absorption of the electrolytic solution (a retained amount of the solution) and make a structure that facilitates the migration and diffusion of the lithium ions by securing the gaps in the active material layer by electrode porosification.
Patent Document 1 discloses an electrochemical battery electrode obtained by adjusting porosity using a thermally decomposable pore-forming material, such as diammonium carbonate. Additionally, Patent Document 2 describes a nonaqueous electrolyte secondary battery electrode having pores formed in a mixture layer formed from a slurry including an active material, a first binder, and a second binder by compressing the mixture layer and then heating the compressed mixture layer to thermally decompose a part of the binders. Patent Document 3 discloses a method of manufacturing a lithium secondary battery electrode including the steps of coating a slurry including an active material, a binding agent, and a resin partially compatible with the binding agent on a current collector surface and drying the slurry.